Battery Leisure Amps

You can but the 80 amp unit will pull the 120 down to it's level.

Normal advice is to install 2 identical batteries...but the above won't cause any other problems if you go that way!

Thanks for the reply

As EJB says, normal practice is to install identical new batteries so that there is a good chance that they will perform in a similar manner, however, this is not ideal. I think it is possible to have two connected and charged separately from, say, a Sargent system and this would be the best but I am new to motorhoming and not so familiar with the systems that are available. 

I'd advise against mixing capacities and ages of batteries in a bank, you won't be gaining any real capacity by joining them all together

You can introduce issues should one of the batteries fail like the others will try to "charge" the failed battery and can boil the electrolyte in it. 

It also has the issue of the additional cabling between the batteries and the extra weight in the battery box (all that extra lead takes a lot of your available payload away)